1. Field of the Invention
This invention concerns a process for the preparation of acetic acid by the direct conversion of carbon monoxide and hydrogen (synthesis gas or syngas) by a specific catalyst system accompanied by a minimum of solvent hydrolysis.
2. Description of the Prior Art
Acetic acid has an important place in the organic chemical industry in that it is the substrate for the production of, amongst others, vinyl acetate which is used in the manufacture of polymers, numerous amino acids, and dietary supplements such as glutamic acid, citric acid and lysine. The commercial processes leading to the production of acetic acid include acetaldehyde oxidation, liquid phase hydrocarbon oxidation, and methanol carbonylation.
With regard to the conversion of syngas to acetic acid, the common current practice is a two-step process in which syngas is converted to methanol over a copper-zinc-aluminum oxide catalyst and the resulting methanol is then carbonylated in the presence of a homogeneous rhodium catalyst.
However, the ever-increasing scarcity of fuel has generated an urgency for the development of chemically more efficient routes to acetic acid which demand the least energy consumption.
A process involving the direct conversion of syngas to a mixture of acetic acid, ethanol and acetaldehyde over metallic rhodium is described in German Pat. No. 2,503,233 to U.C.C. While such a direct route eliminates the methanol formation step, there are several drawbacks to this process including the high cost and limited availability of rhodium, as well as a lack of selectivity to acetic acid.
U.S. Pat. No. 4,366,259 discloses a method for making acetic acid and propionic acid and their esters which comprises contacting a mixture of CO and H.sub.2 with a catalyst system composed of a ruthenium-containing compound and a cobalt halide dispersed in a low melting quaternary phosphonium or ammonium salt at a temperature of at least about 150.degree. C. and at a pressure of about 500 psig, or greater. Optionally, an organic iodide containing compound can be added to the catalyst combination in order to achieve improved selectivity to acetic acid. While selectivity to acetic acid can be improved with the use of the organic iodide compound, such selectivity needs further improvement.